Tanning of hides and skins



Patented Mani 5, 1938 PATENT OFFICE TANNING F RIDES AND SKINS Matthew M- Merrltt,Middleton, Mass, assignorto The Tanning Process Company,

Boston.

Mass.,-a corporation oi Massachusetts No Drawing.

l6 Claims. This invention relates to the tanning of hides and skins and more particularly to chrome tan' ning operations upon lighter skins, such assheep "and kid skins. It is to be understood, however, that the invention and various important characteristics thereof may have other applications and uses.

This application is filed as a continuation in part and as a substitute for the pending applilo cation, Serial No. 691,818, filed October 2, 1933',

in the name of Matthew M. Merritt.

It is an object of the invention to provide improvements in the tanning of hide substance by which 'the operation is speeded up to a substantial' degree, while at the same time an improvedproduct is obtained.

To these ends and in aocordance with an important characteristic of the invention the tanning operation is initiated by subjecting properly 30 prepared hide substance to the combination of a chrome tanning substanceanda tanning agent in itself non-tanning, both of these substances being highly soluble and capable of rapid penetration into hide substance, and subsequently 25 adding to the solution of the above materials a reagent which, in conjunction with said nontanning agent, will secure a tanning effect by setting said tanning agent in and on the fibers of the hide substance. 9 with the said materials the tanning operation is rapidly eifected due to the high speed penetration of the hide substance by the highly soluble chrome tanning substance and the tanning agent which is of itself non-tanning, setting of the at latter in and on the fibers of the hide substance being accomplished by a reagent which is also highly soluble and capable of rapidly permeating the hide substance.

To practice the method, there is provided a 40 chromium salt which is especially soluble, and

rapidly penetrable into hide substance by reason oi the fact that when in solution it has simple chromium nuclei. These latter may be rendered more complex and therefore more effective, as

- 46 a tanning agent, after said chrome tanning substance has fully permeated the hide substance. The tanning composition also contains a chrome tanning agent which is also very soluble and rapidly pe'netrable into hide substance without to producing any tanning effect, this last-mentioned fact accounting in part at least for the rapidity of the penetration of this tanning agent. Another ingredient of the composition is a substance for rendering the described chromium nuclei more complex, thus'giving weight and substance,

By following this method Application April 10, 1936. Serial No. 73,805

to the leather thus produced. Later there is added toe solution of the above-mentioned composition, a. reducing agent by which the chrome tanning agent (which of itself is non-tanning) is causedto combine with the collagen oi the hide 5 substance to produce leather.

In a preferred composition for rapid tanning of hide substance, thereis provided a substantial amount of normal chromic sulphate which is r highly penetrable while in solution into hide sub- 10 stance, apparently because of the fact that this sulphate has chromium nuclei quite simple in structure. With the normal chrcmic sulphate there is associated 'a substantial amount of sodium'dichromate which is also highly soluble and which very readily penetrates hide substance, partly because it does not tan said substance, and which may be subsequently set in and on the fibers of the hide substance by a proper reducing agent. A third constituent of the tanning come 2o position is a soluble sulphate such as sodium sulphate which readily penetrates hide substance and has the effect of rendering the chromium nuclei already in place in the hide substance much more complex, thereby adding to the weight and quality of the tanned leather. For a reducing agent added subsequently to the action of the above-described composition on a given piece of hide substance, there is utilized a suitable quantity of sodium thiosulphate, 'commonly referred to as hypo.

As a matter of convenience, the tanning composition described in the-foregoing paragraphs is prepared by mixing appropriate amounts of basic chromic sulphate, sodium dichromate and sulphuric acid to provide the desired active constituents oi the composition and in amounts most effective to secure rapid and thorough tanning of hide substance.

The above and other important characteristics of the invention will now be described in detail in the specification and then pointed out more particularly in the appended claims.

In tanning operations upon sheepskins, good. results have been obtained from a composition having ingredients in amountsspeclfied as follows:

Kilograms Basic chromic sulphate 4.3 Sodium dichromate 1.2 Sulphuric acid 1.2

In making up the composition, the basic chromic sulphate in it's crystalline form is first mixed with the sodium dichrom-ate also in powdered form, in

a suitable mixer or other container, and then the sulphuric acid is added and the mixing continued for several minutes until a thoroughly uniform mixture is obtained. It is to be noted that the amount of sulphuric acid is about three times that usually provided with the dichromate in the conventional two-bath method of chrome tanning. This excess of acid contributes to the facility with which the chrome salt permeates-the hide substance. I believe that this concentrated acid converts most or all of the basic chromic sulphate into normal chromic sulphate and also part of the dichromate into sodium sulphate. The above measured amount of the composition is wrapped up in a waxed paper package and placed in a canvas bag. Conveniently each package will contain suflicient of the composition to tan five dozen sheepskins of average size, the amount of composition being calculated to take care of the usual variations in the amount of hide substance in different batches of skins. It is to be understood, however, that there may well be some considerable variation from the amounts of the various ingredients as given above due, for

example, to variations in the treatment which the skins receive in the preliminary operations of liming, bating, and pickling. In some bating operations, for instance, all but a fraction of one per cent of the lime is removed, while in other cases as much as two per cent of lime may remain in the skins to be later neutralized by the pickling solution. Hence the proportions of the ingredients in my tanning composition may also be varied to suit different conditions arising from the described preliminary treatment of the raw skins and to suit differences in the qualities and characteristics of the pickled skins or to obtain desired results with different batches of skins intended for a variety of uses. Instead of 4.3 parts of the basic chromic sulphate to 1.2 of the other two constituents of my tanning compound, there may be variation along the lines indicated above to obtain the desired qualities in the tanned skins.

In tanning sheepskins it is preferred to place them in a relatively small drum with enough water to cover the skins nicely, the amount' of water being about 11.3 liters for each five dozen skins, and the temperature being maintained preferably at about F. After a preliminary revolution or so of the drum, the tanning preparation is dropped in, enclosed in one or more canvas bags, as described, and drumming is continued for half an hour. If the described tanning composition were to go immediately into solution or become otherwise dispersed therein, the skins in the drum wouldbe subjected to a tanning solution or dispersion of about 300 Bkr. Since, however, the materials in the canvas bags must dissolve therein and escape therefrom into the body of water in the drum, the solution or dispersion of the ingredients as applied to the skins never reaches the concentratiion of 300 Bkr. but remains substantially below that figure, -for the reason that the skins take up the tanning material very rapidly. If canvas bags are not used, that is, if the described tanning compound is dissolved in water before adding the skins to the drum contents, the amount of water may be increased to lower the barkometer strength of the solution applieddirect to the skins. However, in tanning sheepskins for lining stock the barkometer strength may be maintained at 300 or even increased, in accordance with the results desired and depending on the experience and judgment of the individual tanner. At the end of the hall hour mentioned above, about half of the liquor is drained off and there is then introduced into the drum a reducing agent, thiosulphate of sodium being commonly used in the amount of 8.4 kilograms for each five dozen skins. The drum is then run for another half hour to effect reduction of the dichromate and to fix the chrome 1n and on the fibers of the hide substance. The skins are then washed while in the drum and also drained before removing them from the drum. They are at once slicked out individually upon drying boards and then hung up to dry, the final setting of the chrome material in the hide substance taking place during the drying operation.

The basic chromic sulphate mentioned above has a composition which, upon analysis, may be 1 expressed as follows:

As a result of the reaction between the basic chromic sulphate and the sulphuric acid during and following the mixing of these substances together and their solution in water, it is believed that said basic chromic sulphate is changed largely, if not entirely, to normal chromic sulphate which is highly soluble in water and the molecules of which appear to have simple nuclei containing only one chromium atom each. Because of this simple structure the normal chromium sulphate penetrates hide substance with greater ease and rapidity than any other chromium sulphate. This fact accounts in part at least for the rapid tanning action of my chrome preparation. It is generally understood that while normal chromic sulphate penetrates hide substance with rapidity, the tanning effect is not so pronounced as with basic chromic sulphate and that the normal salt gives a relatively scant tannage. However, the fact is that leather made from sheepskins tanned by my chrome preparation is characterized by a tensile strength far greater than that secured by materials and methods commonly practised in the industry today. Furthermore, this sheepskin leather is remarkable for its firmness and thickness. In other words, it is plump and firm in marked contrast to the rather thin and tinny leather obtained from sheepskins by the commonly practised tanning methods. Because of these facts it is believed that the normal chromium salt, after it has permeated the hide substance, is changed through the action of sodium sulphate present in the described tanning material as a result of the action of sulphuric acid upon the sodium dichromate. This sodium sulphate is highly soluble and in its passage'into the hide substance acts upon the normal chromic sulphate in such manner as to change the simple structure of the nuclei to one much more complex at a time when the normal chromic sulphate is well distributed throughout the hide substance. As a result of this increased complexity in the nuclei of the molecules a very much better tanning effect is produced, resulting in a plumper and firmer leather. As already stated, this explanation of the action of the normal chromic sulphate in my preparation is substantiated by the really excellent results obtained in tanning operations conducted on a large scale under carefully trolled test conditions.

If it be preferred to perform tanning operations upon hides or. skins with a tanning compound comprising the reaction "products described in the preceding paragraph, the proportions will be substantially as follows:

Kilograms Normal chromic sulphate, Cr2(SO4) 3.51120" 3.28 Sodium sulphate, NaaSO4.10H:O "2:06 Sulphuric acid, Tech. 60 Be .59

Sodium dichromate, Tech 1.22

depending, upon variations in the amount of Water of crystallization. While, as stated, the

combination of materials given above has proved to be entirely satisfactory as a chrome tanningpreparation, other salts containing Crz(SO4) 3 and Na2SO4 may be substituted for those given, in which case the proportions of the new ingredients must be adjusted accordingly. The proportions of the various ingredients of thiscombination may be stated as follows, when based upon formulae containing no water of crystallization:

Parts Chromic sulphate, Cr2(SO4)3 about 10 Sodium sulphate, NazSOu, about 3 Sodium dichromate, NazClaOv about 4 Sulphuric acid, H2S04 about 2 It is to be understood that other chromium salts besides the sulphate may be utilized to advantage in quick tanning operations, such for instance as chromlc chloride. Conveniently the tanning operation in that case is begun with a basic chloride of chromium which penetrates hide substance very rapidly. Subsequently sodium sulphate is introduced to render the chromium nuclei in the hide substance more complex thus producing leather having more substance and of decidedly better quality than that produced by chromium chloride alone.

Another reason for the rapid penetration of the described tanning material into the hide substance resides in the fact that the sodium dichromate is also highly soluble in water and readily penetrable into the hide substance, partly at least, because of the fact that it is not, of itself, a tanning agent. After thorough distribution of the sodium dichromate along with, and apparently facilitated by, the normal chromic sulphate throughout the hide substance, the

chromium moleculeis lined in and on the fibersof the hide substance by a reducing agent, the reducing agent being conveniently sodium thicsulphate, as above described. There are certain advantages in applying the described active tanning substances, namely, normal chromic sulphate, sodium dichromate andsodium sulphate,

' to the skins by the indirect route described herein, that is, by introducing into a drum containing a suitable quantity of water a mixture of basic chromic sulphate, sodium dichromate, and sul phurlc acid. The reason for these advantages I have explained as lying, partly at least, in. the.for-

mation of said active tanningsubstances and their interaction while in contact with the skin substance. However, the active tanning agencies formed by the reaction of these substances are, as

stated, normal chromic sulphate, sodium dichromate and sodium sulphate. Hence I claim the latter group of substances more or less broadly as constituting a new composition useful for tanning.

It is preferred to introduce the described tanning composition into the drum in measured packages and in a canvas bag for the reason that the tanning material is dissolved within the bag and is diffused into the surrounding liquid at a rate approximating that at which the skinstake it up, so that the latter are not treated to a tanning liquor at full strength at the beginning of the tanning operation. As pointed out in the foregoing paragraphs, excellent results are obtained by my tanning preparation when properly compounded and applied as directed. It is a demonstrated fact that introduction of basic chromic sulphate, sodium dichromate, and sulphuric acid, in the amounts mentioned, sepa rately into a vessel containing about 11.3 liters of water and the skins to be treated gives very diii'erent results from those obtained by the described mixture of the same substances, as evidenced by relatively slow tanning of the sheepskins and poor quality of the product when the substances are introduced and applied separately to the skins in the tanning drum. However, as stated above, I have found that my' tanning composition maybe placed in solution in a relatively very restricted amount of water and that good results are obtained when sheepskins are tannedwith this tanning liquor in strong or concentrated solution.

it has permeated. the latter.

2. A tanning composition comprising an aqueous dispersion of a non-basic salt of chromium, and of a chrome tanning agent in itself nontanning, both being highly soluble and capable of rapid penetration into hide substance, and said chrome tanning agent being readily reduced in place in and on the fibers of the hidesubstance after it has permeated the latter.

3. A tanning composition comprising a chromic salt, a dichromate in itself non-tanning, both being highly soluble and capableof rapid penetration into hide substance, and sodium sulphate capable in the presence of water of reacting with the chromium nuclei of' said chror'nic salt to render the latter more complex and thus increase the tanning effect of said chromic salt, said dichromate being capableof being reduced in place in and on the fibers of the hide substance after it has permeated the latter.

d. A composition suitable for tanning hides and skins upon the addition of water and comprising normal chromic sulphate, sodium dichromate} both, of which are highly soluble and capable of rapid penetration into hide substance, and sodichromium sulphate, said dichromate being capable of being reduced in place in and on the fibers of the hide substance after it has permeated the latter.

5. A composition suitable for tanning hides and skins upon the addition of water and comprising normal chromic sulphate, sodium sulphate, sodium' dichromate, and a mineral acid.

6. A composition suitable for tanning hides and skins upon the addition of water and comprising chromic sulphate about ten parts by weight, sodium sulphate about three parts, sodium dichromate about four parts, and sulphuric acid about two parts, also by weight.

'7. That improvement in methods of tanning hide substances which comprises subjecting hide substance to treatment by a chrome tanning substance and simultaneously therewith by a chrome tanning agent in itself non-tanning, both being highly soluble and rapidly penetrable into hide substance, and subsequently subjecting said chrome tanning agent in the hide substance to treatment by a reducing agent to "render said tanning agent effective to tan the hide substance.

8. That improvement in methods of tanning hide substance which comprises subjecting hide substance to treatment by a composition comprising a non-basic salt of chromium and a chrome tanning agent in itself non-tanning, both of these substances being highly soluble and rapidly penetrable into hide substance, said composition including also a substance to increase the tanning efiect of the said salt of chromium, and adding a substance to reduce the chrome tanning agent in place in and on the fibers of the hide substance after it has permeated the latter.

9. That improvement in methods of tanning hide substance which comprises subjecting hide substance to treatment by a composition comprising chromic sulphate and sodium dichromate, which in itself is non-tanning, both the sulphate and dichromate being highly soluble and rapidly penetrating the hide substance, said composition including also sulphuric acid to produce sodium sulphate from the sodium dichromate,

the sodium sulphate serving to render the chm-- mic sulphate more effective as a tanning substance, and finally adding a reducing agent to fix the chromium ofthe dichromate in and on the hide fibers.

10. That improvement in methods of tanning hides or skins which comprises subjecting them to treatment by a tanning composition comprising normal chromic sulphate, sodium dichromate, and sodium sulphate.

11. An improved method of making a tanning material which comprises mixing concentrated sulphuric acid and a .basic chromic salt, thereafter adding a dichromate of a metal of the alkali group, and thoroughly mixing the material, the proportions of the various ingredients being such as to result in the formation of a substantial quantity of the normal chromic sulphate 'and of a sulphate of the metal of the alkali group.

12. An improved method of making tanning material which comprises mixing concentrated sulphuric acid, a basic chromic salt, and sodium dichromate, in the proportion of about four parts of the salt to one each of the acid and the dichromate, whereby there is formed a substantial quantity of the normal chromic sulphate and also of sodium sulphate.

13. An improved method of making tanning material which comprises mixing a basic chromic salt, dichromate of sodium, and concentrated sulphuric acid, the proportion of sulphuric acid to the other two constituents being such as to provide an acid reaction in the solution of the tanning material thereby facilitating penetration of the said other two constituents into hide supstance, the sulphuric acid being present in such amount as to produce a substantial quantity of the normal chromic sulphate from the basic chromic salt and also sodium sulphate through its reaction with part of the sodium dichromate, the

purpose of said sodium sulphate being to increase the tanning effect of the normal chromic sulphate..

14. An improved method of making a tanning material which comprises mixing basic chromic sulphate, dichromate of sodium, and concentrated sulphuric acid, in such proportion of the sulphuric acid to the other two constituents that most or all of the basic chromic sulphate is converted into normal chromic sulphate, and part of the sodium dichromate into sodium sulphate, the purpose of the latter being to increase the tanning eflect of the normal chromic sulphate.

' 15. An improved method of making a tanning material which comprises mixing basic chromic sulphate, sodium dichromate, and concentrated sulphuric acid in the proportion of about 150 units byweight of the chromic sulphate to about 40 units by weight of each of the other two constltuents.

16. A tanning material compounded by mixing basic chromic sulphate, sodium dichromate, and concentrated sulphuric acid in the proportion of about four parts or the basic chromic sulphate to one each oithe other two constituents.

MATTHEW M. MERRITT. 

